Update package.xml

Remove the roscpp dependency because it is causing issues with the release of the package on ROS2
8 jobs for pr/63 in 5 minutes and 18 seconds (queued for 1 second)
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  Test
passed doc-coverage #89156

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passed format #89157

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passed robotpkg-py-dynamic-graph-v3-py2-18.04-release #89152

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passed robotpkg-py-dynamic-graph-v3-py3-16.04-release #89150

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passed robotpkg-py-dynamic-graph-v3-py3-18.04-release #89151

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passed robotpkg-py-dynamic-graph-v3-py3-20.04-release #89153

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passed robotpkg-py-dynamic-graph-v3-py3-buster-release #89154

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passed robotpkg-py-dynamic-graph-v3-py3-stretch-release #89155

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